The present invention relates generally to a novel heat resisting cast steel, a steam turbine casing and a manufacturing method thereof, a steam turbine power plant and a steam turbine. More particularly, the invention relates to a heat resisting cast steel which has a high creep rupture strength at a temperature above 621.degree. C. and a good weldability, and is suitable for high pressure and intermediate pressure inner casings, a main steam stop valve and a control valve of an ultra-super critical steam turbine which is operated under a main steam temperature of 621.degree. C. and pressure of 250 kgf/cm.sup.2. The invention also relates to a steam turbine casing, a steam turbine power plant and a steam turbine in which the heat resisting steel is used.
A conventional steam turbine is operated under a condition of a maximum steam temperature of 566.degree. C. and a maximum steam pressure of 246 kgf/cm.sup.2. A material used for the casing is 1Cr--1Mo--1/4V low carbon alloy cast steel or 11Cr--1Mo--V--Nb--N cast steel.
From the standpoint of conservation of fossil fuels, such as petroleum, coal and so on, and energy saving, it is required to improve the efficiency of a thermal power plant. The most effective means for increasing the thermal efficiency is to increase the steam temperature of the steam turbine. However, the strength of the conventional casing materials is insufficient for a highly efficient turbine material, and so a material having a higher strength is required.
In particular, conventional materials have an insufficient high temperature strength to be used for a high temperature steam turbine casing operated at a steam temperature of above 621.degree. C. A casing made of 9%Cr steel is disclosed in Japanese Patent Application Laid-Open No. 7-118812, but it shows a deviation in high temperature strength.
The conventional steam turbine is operated under a maximum steam temperature of 566.degree. C. and a maximum steam pressure of 246 kgf/cm.sup.2.
However, from the standpoint of conservation of fossil fuels, such as petroleum, coal and so on, as well as energy saving and prevention of environmental pollution, it is required to improve the efficiency of a thermal power plant. The most effective means for increasing the thermal efficiency is to increase the steam temperature of a steam turbine. A known rotor material used for a high efficiency turbine is 1Cr--1Mo--1/4V ferritic low carbon alloy forged steel or 11Cr--1Mo--V--Nb--N forged steel, and casing material used for the high efficiency turbine is 1Cr--1Mo--1/4V ferritic low carbon alloy cast steel or 11Cr--1Mo--V--Nb--N cast steel. Further, as for materials having a higher high temperature strength, austenitic alloys are disclosed in Japanese Patent Application Laid-Open No. 62-180044 and in Japanese Patent Application Laid-Open No. 61-23749, and martensitic steels are disclosed in Japanese Patent Application Laid-Open No. 4-147948, Japanese Patent Application Laid-Open No. 2-290950 and Japanese Patent Application Laid-Open No. 4-371551.
As for a material having a high temperature strength higher than that of the above conventional casing materials, an austenitic cast steel has been developed by the inventors of the present invention and is disclosed in Japanese Patent Application Laid-Open No. 61-23749. Although this alloy is excellent in high temperature creep rupture strength, there are problems in that its cost is high and a large thermal stress occurs at starting-up and stopping of the turbine due to a large thermal expansion coefficient.
Although the aforementioned Japanese Patent Application Laid-Opens disclose the rotor materials and casing materials, no attention has been given to the steam turbine, nor to the thermal power plant, operating under a high temperature, as described above.
Further, an ultra-super high temperature and high pressure steam turbine is disclosed in Japanese Patent Application Laid-Open No. 62-248806, but no attention has been paid in this publication to a thermal power plant at all.